1. Field of the Invention
The present invention relates to a vehicle collision detecting system used for activating, for example, an air bag system of vehicles.
2. Description of the Related Art
An air bag system protects passengers in a vehicle by inflating air bags when the vehicle has collided.
FIG. 4 shows a prior art vehicle collision detecting system for such air bag system. In the figure, front impact detecting sensors 1 and 2 are provided at the front portion of a vehicle and impact detecting safety sensors 3 and 4 are provided within a diagnostic unit 5. FIG. 5 is a connection diagram of those sensors 1 through 4, and the sensors 1 and 2 and the sensors 3 and 4 are arranged in parallel respectively and these sensors 1 and 2 and the sensors 3 and 4 are arranged in series to an inflator 6.
The front sensors 1 and 2 are switched ON when an acceleration of, for example, more than 10 to 15 G (G: gravitational acceleration) is detected and the safety sensor 3 and 4 are switched ON when an acceleration of 2 to 3 G is detected. The safety sensors 3 and 4 are provided to determine whether an impact is a collision and to expand an air bag 7 only when both the sensor 1 or 2 and the sensor 3 or 4 are switched ON, because the sensors 1 and 2 may be switched ON even by such a degree of impact as hit by a hammer.
Japanese Patent Laid-open NO. Hei 2-218988 describes a method for predicting a collision by dividing a distance from a foregoing vehicle by a difference of relative speeds with the foregoing vehicle.
In the aforementioned prior art air bag system, however, the inflator 6 is not ignited and activated during the time period between when the sensor 1 or 2 is switched ON and when the sensor 3 or 4 is switched ON, so that the expansion of the air bag is delayed to that extent. Although an air bag for the driver's seat may be expanded so that a driver is fully protected using a small capacity inflator even if the activation of the inflator is delayed more or less since the volume of the air bag is small, the air bag for the assistant driver's seat requires an inflator having a large capacity, since the volume of the air bag is large, to rapidly inflate the large air bag to securely protect the passenger. Accordingly, it has become a factor for increasing the cost of the system.
By the way, in order to rapidly expand the air bag, the air bag has to be strong so that it can sustain an impact applied thereto when the expansion is completed. Accordingly, it has become another factor for increasing the cost.
The aforementioned method for predicting a collision by dividing a distance from a foregoing vehicle by a difference of relative speeds cannot be utilized for the vehicle collision detecting system for the air bag system as it is since its prediction reliability is too low.